This invention relates to an impingement drying unit for a dryer section of a machine making a fibrous web, advantageously a paper or board machine, which comprises at least one drying group and an impingement drying unit in which a fibrous web, advantageously a paper or board web, i.e. a web, has been arranged to be conducted past the impingement drying unit while supported by a drying fabric, such as a wire or equivalent.
The invention also relates to a dryer section of a machine making a fibrous web, advantageously a paper or board machine, which comprises at least one drying group applying single-wire draw or twin-wire draw, and an impingement drying unit for drying a fibrous web, advantageously a paper or board web, i.e. a web, and a drying fabric, such as a wire or equivalent on whose support the web has been arranged to be conducted past the impingement drying unit.
As known from the prior art, multi-cylinder dryers of paper machines employ twin-wire draw and/or single-wire draw. In twin-wire draw, the groups of drying cylinders comprise two wires which press the web, one from above and the other one from below, against heated cylinder surfaces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering, which may cause web breaks, in particular as the web is still relatively moist and, therefore, has a low strength. Therefore, ever increasing use has been made of said single-wire draw in which each group of drying cylinders comprises only one wire on whose support the web runs through the entire group so that the wire presses the web against the heated cylinder surfaces of the drying cylinders and the web remains at the side of the outside curve of the turning cylinders or rolls situated between the drying cylinders. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the turning cylinders or rolls are arranged inside the loop. One problem in these prior-art arrangements has been presented by the cylinders and rolls which are situated outside the wire loop and which get soiled, wherefore it has been necessary to provide them with doctors to keep the surfaces clean.
With increasing speeds of paper machines, the runnability of a paper machine is, of course, also affected by the dryer section, whose length with the prior-art multi-cylinder dryers would, at high speeds, become very long. If it is imagined that a present-day multi-cylinder dryer were used at a web speed of 40 m/s, it would include about 70 drying cylinders, and its length in the machine direction would be about 180 m. In that case, the dryer would comprise about 12–15 separate wire groups and a corresponding number of draws over the group gaps. It is to be assumed that, in a speed range of 30–40 m/s, the runnability of the normal prior-art multi-cylinder dryers is not good because of open draws, which may additionally cause web breaks that reduce the efficiency of the paper machine. In a speed range of 30–40 m/s and at higher speeds, the prior-art multi-cylinder dryers applying single-wire draw would also become uneconomical because the cost of investment of an excessively long paper machine hall would be high.
A dryer section like the one shown in FIG. 1 is previously known from public FI patent application 981331, in which dryer section one of the cylinder drying groups in which a web runs on support of a wire, comprises an impingement drying unit 10 placed in the basement, i.e. below the machine level or machine line parallel to the horizontal plane of the paper machine, which impingement drying unit includes a large-diameter cylinder 12 around which a wire 1 and a web 2 run, and in which impingement drying unit air impingement acts directly on the surface of the web 2. The total length of the dryer section can be shortened to some extent by this kind of impingement drying unit because the necessary drying capacity can be placed in a shorter space in the machine direction (MD).
FIG. 1 illustrates an example in which in the space required by three drying cylinders typically with a diameter of 1.8 m there are two drying cylinders with partial coverage and impingement below the machine level, the diameter of the impingement cylinder being 3.6 m and the coverage of air impingement being about 200°. In that connection, as compared with the drying capacity of the drying that applies single-wire draw, a higher drying capacity is achieved from the same horizontal space, i.e. the space taken by three drying cylinders, because of the impingement drying placed below.
Generally, the numerical relationship of drying capacities, expressed by the number of drying cylinders, can be estimated from the formulaXcyl/(Ycyl+Zcyl),which, as applied to the above-mentioned exemplifying case, is3 cyl/(1.5 cyl+Π*3.6/1.7*200°/360°)cyl≈about ⅗,where 1.5 cyl is the drying capacity of about two drying cylinders with a smaller coverage than normal, and 1.7 follows from the fact that, on average, an impingement length of 1.7 m corresponds to the capacity of a 1.8-m diameter drying cylinder with normal coverage. Thus, in this numerical exemplifying case it is possible to achieve an additional drying capacity that corresponds to the drying capacity of about 2 drying cylinders. It must be emphasised that the large-diameter cylinder 12 situated below takes a considerable space in the direction of the machine line and the revolving mass increases with the large diameter of the cylinder.
The other problems which are manifested with ever higher emphasis at high speeds of the paper machine and for which satisfactory solutions have not yet been found, at any rate not for all of them, include the quality problems associated with the uniformity requirements of both the machine direction and the cross direction profiles of the paper web. The uniformity of the web being produced also influences the runnability of the entire paper machine and it is also an important quality factor of finished paper. Implementation of high speeds, in particular in wide machines, thus poses ever more difficult problems to be solved, of which the most important ones are the runnability and sufficient dewatering capacity of the machine at high speed.
One problem in paper machine dryer sections, when using conventional dryer sections composed of multi-cylinder dryers, is the noise caused by them. In addition, when cylinder drying is used, it is necessary to use steam, in which connection the drying cylinders must be designed to comply with pressure vessel regulations and, furthermore, the hood placed around the dryer section must be thermally insulated.
With respect to the state of the art relating to the invention, reference is made to FI patent 102623, which discloses a method in a paper machine or equivalent in which a web is dried in at least one drying group based on impingement drying or equivalent. In this prior-art arrangement, the web is guided along a substantially linear path or by using a large constant curve radius and in the drying stage, after impingement drying, the web or equivalent is dried in at least one drying group applying normal single-wire draw, and the web is passed from a pressing stage to the drying stage as a closed draw and so that the web is constantly supported against at least one support surface. The above-mentioned patent describes different types of applications for placing impingement drying units in a dryer section and in connection with arrangements of this type, reference is also made to the magazine article Markus Oeschle: “Drying concepts for high demands”, Paperi ja Puu, Vol. 81, No. 8/1999.